1. Field of the Invention
This invention generally relates to a material structure with a transparent conductive oxide (TCO) layer and, more particularly, to a process for forming a textured TCO layer to increase light confinement, suitable for solar cells.
2. Description of the Related Art
Transparent conducting films (TCFs) for photovoltaic applications are fabricated from both inorganic and organic materials. Inorganic films typically are made up of a layer of TCO (transparent conducting oxide), generally in the form of indium tin oxide (ITO), fluorine doped tin oxide (FTO), or doped zinc oxide (ZnO). Transparent conducting films act both as a window for light to pass through to the active material beneath (where carrier generation occurs) and as an ohmic contact for carrier transport out of the photovoltaic. Transparent materials possess bandgaps with energies corresponding to wavelengths which are shorter than the visible range (380 nanometers (nm) to 750 nm). As such, photons with energies below the bandgap are not collected by these materials and visible light passes through. However, applications such as photovoltaics may require an even broader bandgap to avoid unwanted absorption of the solar spectra.
Transparent conductive oxides (TCO) are metal oxides, often doped, that may also be used in flat panel displays, as well as photovoltaic devices. Most films are fabricated with polycrystalline or amorphous microstructures. On average, these applications use electrode materials that have greater than 80% transmittance of incident light as well as high conductivities for efficient carrier transport. The transmittance of these films, just as in any transparent material, is limited by light scattering at defects and grain boundaries. In general, TCOs for use as thin-film electrodes in solar cells should have a minimum carrier concentration on the order of 1020 cm−3 for low resistivity and a bandgap less than 380 nanometers to avoid absorption of light over most of the solar spectra. Mobility in these films is limited by ionized impurity scattering and is on the order of 40 cm2/(V·s). Conventional transparent conducting oxides used in industry are primarily n-type conductors, meaning their primary conduction is from the flow of electrons. Suitable p-type transparent conducting oxides are possible.
To date, the industry standard in TCO is ITO. This material boasts a low resistivity of ˜10−4 Ω·cm and a transmittance of greater than 80%. However, ITO has the drawback of being expensive. Indium, the film's primary metal, is rare. For this reason, doped binary compounds such as aluminum-doped zinc-oxide (AZO) and indium-doped cadmium-oxide have been proposed as alternative materials. AZO is composed of aluminum and zinc, two common and inexpensive materials, while indium-doped cadmium oxide only uses indium in low concentrations.
Binary compounds of metal oxides without any intentional impurity doping have also been developed for use as TCOs. These systems are typically n-type with a carrier concentration on the order of 1020 cm−3, provided by interstitial metal ions and oxygen vacancies which both act as donors. However, these simple TCOs have not found practical use due to their electrical properties' high temperature and oxygen partial pressure dependence.
Doped metal oxides for use as transparent conducting layers in photovoltaic devices are typically grown on a glass substrate. This glass substrate, apart from providing a support on which the oxide can grow, has the additional benefit of blocking most infrared wavelengths greater than 2 μm for most silicates, and converting it to heat in the glass layer. This in turn helps maintain a low temperature of the active region of the solar cell, which degrades in performance as it heats up. TCO films can be deposited on a substrate through various deposition methods, including metal organic chemical vapor deposition (MOCVD), metal organic molecular beam deposition (MOMBD), spray pyrolysis, and pulsed laser deposition (PLD). However, conventional fabrication techniques typically involve magnetron sputtering of the film. The sputtering process is very inefficient, with only 30% of the material actually being deposited on the substrate. In the case of ITO this inefficiency is a significant drawback. Growth typically is performed in a reducing environment to encourage oxygen vacancy formation within the film, which contributes to the carrier concentration (if n-type).
Charge carriers in these oxides arise from three fundamental sources: interstitial metal ion impurities, oxygen vacancies, and doping ions. The first two sources always act as electron donors. Indeed some TCOs are fabricated solely using these two intrinsic sources as carrier generators. When an oxygen vacancy is present in the lattice it acts as a doubly-charged electron donor. In ITO for example, each oxygen vacancy causes the neighboring In3+ ion 5s orbitals to be stabilized from the 5s conduction band by the missing bonds to the oxygen ion, while two electrons are trapped at the site due to charge neutrality effects. This stabilization of the 5s orbitals causes a formation of a donor level for the oxygen ion, determined to be 0.03 eV below the conduction band. Thus, these defects act as shallow donors to the bulk crystal. To enhance their electrical properties, ITO films and other transparent conducting oxides are grown in reducing environments, which encourages oxygen vacancy formation.
Dopant ionization within the oxide occurs in the same way as in other semiconductor crystals. Shallow donors near the conduction band (n-type) allow electrons to be thermally excited into the conduction band, while acceptors near the valence band (p-type) allow electrons to jump from the valence band to the acceptor level, populating the valence band with holes. Carrier scattering in these oxides arises primarily from ionized impurity scattering. Charged impurity ions and point defects have scattering cross-sections that are much greater than their neutral counterparts. Increasing the scattering decreases the mean-free path of the carriers in the oxide, which leads to poor device performance and a high resistivity. An insulator such as an oxide can experience a composition-induced transition to a metallic state given a minimum doping concentration, permitting carrier flow.
Two other TCOs that are often used are ZnO/Al and In2O3/Sn. In materials science, ZnO is often called a II-VI semiconductor because zinc and oxygen belong to the 2nd and 6th groups of the periodic table, respectively. This semiconductor has several favorable properties: good transparency, high electron mobility, wide bandgap, strong room-temperature luminescence. ZnO has a relatively large direct band gap of ˜3.3 eV at room temperature. The advantages associated with a large band gap include higher breakdown voltages, the ability to sustain large electric fields, lower electronic noise, and high-temperature and high-power operation.
Most ZnO has n-type character, even in the absence of intentional doping. Nonstoichiometry is typically the origin of n-type character. Controllable n-type doping is easily achieved by substituting Zn with group-III elements such as Al, Ga, In, or by substituting oxygen with group-VII elements chlorine or iodine.
In thin-film solar cells, light confinement techniques increase the path traveled by the incoming light, therefore, increasing the probability of photogeneration per incident photon. Light confinement is different from anti-reflection coating, which increases the fraction of photons admitted to the cell. Light confinement, or light trapping, can be achieved by texturing the rear surface, such that the reflected light rays from the rear surface reach the front surface at a lower angle. The rays are unable to escape from the cell and are reflected back in multiple paths between the front and rear surfaces. Light trapping is an essential technique for realizing high efficiency, high stability, and low cost, which are all required for the practical application of thin-film solar cells.
In the past decade the use of transparent conductive oxides as front and/or back contact material in thin-film solar cells has been studied in detail. In recent years ZnO and doped ZnO films have emerged as one of the most interesting materials. Various methods to form ZnO films, and the techniques to etch and control the surface texture, have been explored in great depth. However, most developed techniques still require expensive vacuum equipment, and the process is still complex. Large area ZnO films for high efficient silicon thin-film modules are mostly prepared by high rate sputtering. The cost of production is still high. Chemical vapor deposition of ZnO requires vacuum pumps, high vacuum chamber, and gas flow and pressure control. Although simpler methods of chemical vapor deposition, such as combustion chemical vapor deposition (CCVD) and aerosol chemical vapor deposition (Aerosol CVD) have been reported, more thorough evaluations of the films for high efficient reliable and low cost solar modules are lacking.
Most deposited films show some degree of surface texture. The surface roughness or undulation of ZnO films can be enhanced by adjusting the deposition condition. However, an additional etch step is needed to generate reproducible surface texturing. Many have reported the use of dilute HCl solution to obtain textured ZnO, where the roughness has been found to increase with etch time. The relative high etch rate of HCl solution, even in dilute solution, makes etch control difficult. Aqueous solutions of NH4Cl have also been tested and found to generate textured surface morphology. Most of the existing methods to form ZnO films involve vacuum process, an additional etch process is added to generate the surface texture. The cost could be greatly reduced if ZnO and doped ZnO films with textured could be generated at the same time by a simple solution process or using printing technology.
In the past few years, studies using solution processes to generate ZnO layer have been reported from several institutes. Most of the reports are focused on the semiconductor behavior of the film, for thin-film transistor (TFT) in display application. In this area, the ZnO films need to meet the need of good mobility, high transistor on/off ratio, and well-controlled threshold voltage. The surface morphology, high conductivity, and optimized optical transmission are not the main concerns, and the control of surface texture is not discussed in these reports.
It would be advantageous if metal oxide films, such as ZnO and doped ZnO films, could be properly textured for light trapping, using inexpensive solution and printing technologies.